Yesterday I noticed a knocking coming through the pedals at high speed. It wasn’t always there but was noticeable when it was. I’m assuming big end bearing or timing belt, so I’m replacing both as I don’t think either have ever been done, but is there anything else I should be looking at just in case, that could cause it
That seems like a lot of work to do off the back of an assumption. It’s not very often that big end failure or timing belt noise is described as being felt through the pedals, at high speed. I’d do a bit more investigating first. Ca you hear any noise from the engine itself if you increase and decrease the revs with the car stationary?
I feel it in the pedals and on the foot rest by the clutch and hear it in the cabin.
I’ll try revving the car when stationary and see if I hear/feel it. Thanks 
Discs??
Spent a few minutes revving the car today and didn’t notice the knock. Will try taking it out for a run out later this week and see if I notice it again, then try revving while warm again.
When you brake from speed, does the pedal pulse?
No, it happens when maintaining speed. It’s not really a pulse, just I can feel that something is knocking, through the pedals, floor and foot rest.
If you lightly touch the brake pedal whilst moving - does the noise/knocking stop ?
A mate of mine has just had similar and it ended up being the aftermarket front pads he was using.
They were rattling in the calipers.
A change to a set of Mintex pads stopped it.
It’s highly unlikely that the timing belt would manifest to knock you could feel, and big end bearings would be game game over for the engine, the engine would keel over and die very shortly after hearing/feeling it, so I’ll suggest that you’re barking up the wrong tree.
However changing the timing belt would be a wise choice if you’ve no record of its age.
I think the first thing I’d try is coasting. Maybe dip the clutch or knock it out of gear and see what happens.
Loose undertray??
Another update. Knocking was there again last night. Only seems to happen in motion, not when stationary.
Also started getting a squealing when accelerating last night, so definitely going to change fan and timing belts, just for my own sanity.
Feel like the knocking is something wheel related.
Could it be an engine or gearbox mount if it only does it whilst in motion, or even a loose bracket / bolt?
rreckless - I always had problems with the ‘squealing’ on my Mk2 - I tried different belts and tensions, a new battery, even a new alternator, but it always recurred. I finally got a trickle-charger, the type that you plug into a connector permanently attached to the battery, which I plug-in every time I’m home, and I haven’t had a squeal since. Admittedly not much use if you park on the street, but if you can park on your own property it might be worth trying.
Having read some more I think it might be engine mounts. I noticed a bit of a rock in the car when revving the stationary car. From what I’ve read it can also cause parts to rub which may explain the squeal too.